The correct Active/Passive form of the given sentence is:

☀ He is believed to be a spy.

Explanation for other options:

☀ He is believing to be a spy. This option is not in the correct passive voice. It uses the active voice "is believing" instead of the passive "is believed."

☀ He had been believed to be a spy. This option uses the past perfect tense ("had been believed"), which suggests an action completed before a certain point in the past. The original sentence is in the present tense, so the correct form should be "He is believed to be a spy."

☀ He was believed as to be a spy. This option is not in the correct passive voice. It uses the active voice "was believed" instead of the passive "is believed." Additionally, the use of "as to be" is not idiomatic in this context. The correct form should be "He is believed to be a spy."